The present invention relates to a method of manufacturing a vehicular traction apparatus, which is suited to, for example, an automobile traction apparatus and mounted with a hook portion of a traction hook in a predetermined orientation to a connecting member on a side of an automobile, and in which generation of rolling and looseness of the traction hook by vibrations and an external force at the time of traction is prevented, reliability and safety in traction are ensured, the traction hook is smoothly mounted to the connecting member, and it is possible to increase the parts in strength, to make them small in size and weight, to facilitate processing them, and to manufacture them at low cost, and a vehicular traction hook, and a method of manufacturing a vehicular connecting member.
A conventional automobile traction hook is mounted to a vehicle body to be used at the time of shipping by a ship or the like for automobiles or traction and comprises a male thread portion provided at one end of a shank portion made of, for example, steel and a hook portion provided at the other end thereof to enable a wire rope or the like to be latched thereon, the male thread portion is screwed into a female thread portion of a hook stay fixed behind a bumper in use to be clamped, one end of the wire rope is latched on the hook portion, and the other end of the wire rope is mounted to a pulled side or an immovable portion (see, for example, Patent Document 1).
Since the male thread portion is worked irrespective of a mount position or orientation of the hook, however, the hook is diversified in mount position or orientation when the male thread portion is screwed into the female thread portion to be fixed, thus causing a problem that at the time of traction of an automobile with the use of the hook portion, a bending moment or vibrations generated by the wire rope as set loosen a part of the hook portion to cause looseness or rolling to lead to decrease traction in safety and reliability.
In particular, in case of shipping an automobile by a ship, while traction hooks are mounted to front and rear portions of the automobile, lengths of wire rope are set on the hooks, and one ends of the lengths of wire rope are moored immovably to the ship, the automobile moves following a complex behavior at the time of navigation and thread portions of parts of the hooks are loosened by a bending moment or vibrations generated by the wire rope and further acted repeatedly by a complexly acting force, thus causing a problem that the looseness or the bending moment increases to cause a damage to mooring in safety and so shipping of the automobile is damaged in safety and reliability.
When looseness is generated in the hook portion, a larger stress than that at an ordinary occasion is generated on the thread portions to give rise to a fear of insufficiency thereof in strength, so that in anticipation of looseness in the thread portions, there is adopted measures of an increase in strength, such as an increase in diameter of the thread portions.
Accordingly, this case involves a problem that traction hooks are made large in size and weight and correspondingly take much time in manufacture to become expensive or the like.
Hereupon, in order to solve such problem, there is conceivable a method of subjecting a traction hook and a connecting member, into which the traction hook is screwed, to forged forming to increase the same in strength, subjecting thread worked portions to pre-working with accuracy, and cutting peripheral surfaces thereof with dies, or rolling to work thread portions.
In this case, however, there is a disadvantage that much time is taken in working of thread portions to cause an increase in manufacturing cost, even when a traction hook and a connecting member are specially subjected to forged forming, grain flows are parted by cutting working or rolling working and a predetermined strength by the grain flows is lost.
Also, with the thread working, working is impossible to make a mount position or orientation of a hook fixed, and so a fear in safety at the time of traction cannot be eliminated.
Therefore, with a view to solving the problem, there exists a forming method, in which a core formed on an outer periphery thereof with a male thread portion is made of a hard metal being a sintered alloy, in a state, in which the core is inserted into a hole formed on a metallic, forming raw material, the forming raw material hot forging by upper and lower dies, forged forming is used to form female threads on the hole, then one of the forged part and the core is rotated to permit the core to be pulled out from a female thread portion, and forged forming is used to form female threads on an inner surface of the hole of the forming raw material (see, for example, Patent Document 2).
Also, a lower die having a cavity formed with a female thread portion is made of a hard metal being a sintered alloy, a metallic, forming raw material is inserted into the cavity, the raw material is subjected to hot forging by upper and lower dies, forged forming is used to form male threads on a peripheral surface of the raw material, then a forged part is pulled out from the female thread portion with the upper die removed and the metallic die locked in rotation, and male threads are formed on the peripheral surface of the raw material (see, for example, Patent Document 2).
Since the conventional thread part is subjected to thread forming irrespective of a position, in which it is clamped, however, the clamped position is diversified and so cannot be adopted for an automobile traction hook.
Besides, with the method of forged-forming female threads, it is difficult to have the upper and lower dies pushing a single forming raw material to definitely and uniformly form female threads on the inner surface of the hole, and with the method of forged-forming male threads, the metallic die is pushed in an axial direction of the forming raw material to cause a problem that a large capacity is required of a forging machine to lead to a high installation cost.
Hereupon, with a view to solving the problem, there exists a method of forming male threads, in which method a pair of metallic dies provided on an inner surface thereof with a portion having screw teeth and a portion having no screw teeth are arranged opposedly, a forming raw material as heated is arranged between the metallic dies, the metallic dies are caused to approach and move to form screw teeth and non-screw teeth on a peripheral surface of the raw material (see, for example, Patent Document 3).
Since the conventional thread part is subjected to thread forming irrespective of a position, in which it is clamped, however, the clamped position is diversified and the thread portion is triangular in cross section, small in strength, and large in screw pitch to be liable to cause looseness, so that it cannot be adopted for an automobile traction hook, on which a large load is applied and for which it is essential to ensure safety.    Patent Document 1: JP-A-2003-2136    Patent Document 2: JP-A-8-90138    Patent Document 3: JP-A-2000-202567